In an image display apparatus such as a liquid crystal display, a shift register for performing a shift operation in one frame period of a display signal can be used as a gate-line driving circuit (scanning-line driving circuit) for scanning the display panel. It is desirable that this shift register be formed by field effect transistors of the same conductivity type only in order to reduce the number of steps in the manufacturing process of the display apparatus.
A display apparatus employing amorphous silicon thin-film transistors (hereinafter referred to as “a-Si transistors”) as shift registers of the gate-line driving circuit easily achieves large-area display with great productivity, and is widely used as the screen of a notebook PC, a large-screen display apparatus, etc.
On the other hand, it is known that a-Si transistors are characterized in that the threshold voltage is shifted when the gate electrode is continuously (dc-) biased. In addition, as shown in “Relative importance of the Si—Si bond and Si—H bond for the stability of amorphous silicon thin film transistors” by R. B. Wehrspohn etc., Journal of applied physics, vol. 87, pp. 144-154, a-Si transistors generally undergo progressive degradation.
Further, since the shift of threshold voltage (Vth shift) of transistors causes malfunctions of the circuit, various types of shift registers having taken measures against such malfunctions have been proposed (e.g., Japanese Patent Application Laid-Open No. 2006-107692). It is known that this Vth shift problem similarly occurs in organic transistors, not only in a-Si transistors.
The threshold voltage of an a-Si transistor shifts in the negative direction with the lapse of time when the potential condition where the gate continues having a lower potential than both the drain and source continues. When the threshold voltage of the a-Si transistor shifts in the negative direction, reducing the gate-source potential to turn off the a-Si transistor (into non-conducting state) will not bring about a complete off state. That is, the a-Si transistor cannot interrupt current completely, which causes malfunctions of the circuit.
A shift register includes an output pull-up transistor (Q1 in FIG. 1 of the present application) supplying a clock signal to an output terminal to pull up the potential of the output terminal and a charging transistor (transistor Q3 in the same drawing) for charging the gate node (node N1 in the same drawing) of the output pull-up transistor. In the normal operation of the shift register as will be described later in detail, the charging transistor is in the aforementioned potential condition (where the gate of the transistor has a lower potential than both the drain and source) for a certain period of time, disadvantageously causing the negative shift of threshold voltage, which results in malfunctions.